1. Field of the Invention
A present invention relates to a hologram manufacturing apparatus and a hologram manufacturing method. More specifically, the invention relates to a step and repeat exposure method used in the hologram manufacturing apparatus and method of manufacture.
2. Related Background Art
A hologram optical element is generally a diffraction grating which is manufactured in a manner to record an interference fringe due to light on a photosensitive material. The element is used for the purpose of condensing, focusing and spectrometering light, and includes a hologram lens, a holographic headsup display and a holographic helmet mounted display. In the prior art, the hologram optical element has been manufactured using a two-beam exposure method. However, when the hologram optical element manufactured by the two-beam exposure method is used as a focusing element, aberration occurs and a correction of the aberration is needed.
As one method of correcting the aberration, a hologram manufacturing method for manufacturing a hologram optical element by a step and repeat exposure method has been proposed. In this hologram manufacturing method, a hologram plane is divided into a plurality of appropriate areas in a design step of the hologram optical element. An optimum exposure optical system is designed for each area. An aperture for each area for control of light exposure is formed by a light shielding plate having an aperture of a specific shape. Finally, a laser light source is moved to an optimum position. Those steps are repeated. This hologram manufacturing method increases freedom of design, and it is considered a hologram manufacturing method which greatly contributes to the improvement of the aberration characteristics of a hologram optical element.
FIG. 1 shows a schematic configuration of a prior art hologram manufacturing apparatus for manufacturing a hologram optical element by the step and repeat exposure method.
This hologram manufacturing apparatus is disclosed in U.S. Pat. No. 3,807,829 and comprises a fixing table 101, an L-shaped lateral turntable 103 having a bottom thereof mounted on the fixing table 101 through a lateral turning member 102. The apparatus also comprises a longitudinal turning member 104 mounted on a side of the lateral turntable 103, and an L-shaped exposure table 105 having a side thereof mounted on the longitudinal turning member 104. Two apertured holders 106.sub.1 and 106.sub.2 are fixed to the exposure table 105 to face each other, and a photosensitive material 107 is sandwiched and fixed between the two apertured holders 106.sub.1 and 106.sub.2. When the photosensitive material 107 is to be exposed to light, a laser object light LB is deflected by first and second movable mirrors 108 and 109 so that it is always directed to a beam expander 110. The laser object light LB after being condensed by the beam expander 110 is directed to the photosensitive material 107 through a pin-hole 111. An angle of incidence of the laser object light LB to the beam expander 110 at the second mirror 109 is adjusted by two angle adjustment knobs 112.sub.1 and 112.sub.2. The beam expander 110 and the pin-hole 111 are mounted on a movable stage 113 which is movable by two coarse adjustment knobs 114.sub.1 and 114.sub.2 and three fine adjustment knobs 115.sub.1 to 115.sub.3 so that the laser object light LB can be directed to any point on the photosensitive material 107. A reference light (not shown) is directed to the photosensitive material 107 from the opposite side to the beam expander 110 and the pin-hole 111.
In this hologram manufacturing apparatus, the laser object light LB and the reference light interfere with each other on the photosensitive material 107 so that an interference fringe is recorded on the photosensitive material 107. The area of the photosensitive material 107 that is irradiated by the laser object light LB and the reference light is smaller than the total area of the photosensitive material 107, and the exposure is repeated while the exposure area of the photosensitive material 107 is changed in a stepwise manner. In this manner, the interference fringe is recorded on the entire photosensitive material 107.
In the prior art hologram manufacturing apparatus, however, in order to change the exposure area of the photosensitive material 107 in a stepwise manner, the exposure table 105 is rotated by the lateral turning member 102 and the longitudinal turning member 104 to change the angle of incidence of the laser object light LB and the reference light to the photosensitive material 107. The first and second mirrors 108 and 109 are rotated to change the essential point light source position of the laser object light LB (same for the reference light), and the movable stage 113 is moved horizontally and vertically relative to the exposure table 105 to change the area irradiated by the laser object light LB (same for the reference light). The apparatus has the following problems.
(1) Since the area irradiated by the laser object light LB and the reference light is moved by mechanical parts and the photosensitive material 107 is also driven, a positioning error due to vibration easily occurs.
(2) Freedom of design is small because the shapes of the apertured holder 106.sub.1 and 106.sub.2 are not easy to change and a complex shape such as a circle or oval is not easy to make.
(3) Since the irradiated area cannot be successively removed, discontinuity easily occurs in the interference fringe of the hologram at the boundary of the irradiation areas of the laser object light LB and the reference light.